A METHOD AND APPARATUS FOR TESTING THE THERMAL STABILITY OF AN (AVIATION) FUEL

Abstract: A method and apparatus for testing the thermal stability of an (aviation) fuel. Under pressure a fuel sample to be tested is supplied to a small-bore capillary tube (2) of a test section and the fuel in the test section is allowed to be heat stressed. The fuel sample in the test section is under turbulent flow regime. The thermal degradation of the test fuel within the section is monitored by measuring by means of contactless thermometry the outer wall temperatures along the test section and subsequent increases in these temperatures, promoted by the insulation effect of fuel degradation products. From the temperature data, thus obtained, information on the thermal stability of the fuel is derived.

Description: A METHOD AND APPARATUS FOR TESTING THE THERMAL STABILITY OF AN (AVIATION) FUEL The present invention relates to a method and apparatus for testing the thermal stability of a fuel, e.g. aviation gas-turbine or jet fuel. In particular, the invention relates to small scale turbulent flow testing of (jet) fuel thermal stability. Modern aircraft subject aviation turbine fuel to a range of thermal stresses. This results from the use of fuel as a sink for waste heat and the fact that certain components of the fuel system are located .in regions of high ambient temperature. The heat thus acquired by the fuel stimulates oxidation reactions that lead to the formation of insoluble material. This is clearly undesirable and potentially harmful if, for example, lacquers form on heat exchanger and control surfaces, thereby reducing their efficiency. With respect to heat/thermal stressing of fuel the following points are of interest: a) The heat stress in aircraft and test rigs is under pressure (greater than 20 atmospheres) to ensure that the fuel does not vaporise but always remains in the liquid phase. b) Temperatures in aircraft and test rigs are high enough for fuel oxidation reactions to predominate, but not high enough for fuel pyrolysis reactions to predominate. (c) Heat stressing fuel in a real aircraft will take the fuel to 130-150 °C in the hottest heat exchanger (the oil-cooler) and to 180-200 °C in the feed-arm just prior to the atomiser in the combustion zone. In addition, the fuel insolubles can obstruct or even block parts of the fuel system, for example, the atomizer head in the combustion zone. The continuing trend toward higher engine operating temperatures means that there is an increasing likelihood of problems being encountered as a result of thermally induced deposit formation. A small-scale test configuration has already been proposed, wherein smooth small-bore tubes are applied, which are electrically heated, and under turbulent flow conditions similar to that which is produced in the engine heat exchangers. However, these known test rigs require large fuel flows and quantities (e.g. 2000-80000 litres of fuel per test). Further, in the known test rigs the outer wall temperature or thermal resistance of the tube is established to gauge the quantity of fuel lacquer deposited. In these known test rigs a single or two thermocouple measurements are used, thus obtaining a single measure of temperature or some average figure. However, fuels will not lay down lacquer layers of even thickness along the length of the test tube, but will produce a distribution of lacquer thickness. Moreover, this pattern of lacquer distribution is fuel dependent and unpredictable. The above systems fail to recognize the important point of lacquer distribution and as a result they could fail to distinguish two fuels of different stability. Thus, in order to avoid operational problems, it is essential to have a realistic test method and apparatus. i is an object of the present invention to provide a method and small-scale apparatus for testing the thermal stability of a fuel under turbulent flow conditions, wherein the fuel flow is reduced, e.g. to about 5 litres per test only, which is useful as often in reality only small samples are available. it is another object of the invention to provide a method and apparatus for assessing a liquids heat stability in a turbulent flow regime, which also recognises and allows the full lacquer pattern to be determined. The invention therefore provides a method for testing the thermal stability of a fuel comprising the steps of a) supplying under pressure a fuel sample to be tested to a small-bore capillary tube of a test section and allowing the fuel in the test section to be heat stressed; b) ensuring that the fuel sample in the test section is under turbulent flow regime; c) monitoring the thermal degradation of the test fuel within the section by measuring by means of contactless thermometry the outer wall temperatures along the test section and subsequent increases in these temperatures, promoted by the insulation effect of fuel degradation products; and d) deriving from the temperature data, thus obtained, information on the thermal stability of the fuel. The invention further provides an apparatus for testing the thermal stability of a fuel comprising means for supplying under pressure a fuel sample to be tested to a small-bore capillary tube of a test section and means for allowing the fuel in the test section to be heat stressed; means for ensuring that the fuel sample in the test section is under turbulent flow regime; means for measuring by means of contactless thermometry the outer wall temperatures along the test section and subsequent increases in these temperatures, promoted by the insulation effect of fuel degradation products; and means for deriving from the temperature data, thus obtained, information on the thermal stability of the fuel. Advantageously, the contactless thermometry is controlled scanning I.R. (infra-red) thermometry. The invention will now be described in more detail by way of example by reference to the accompanying drawings, in which fig. 1 represents schematically an embodiment according to the present invention; and fig. 2 represents a three dimensional temperature profile of the capillary test section, obtained according to the invention. Referring now to fig. 1, a capillary test section 1 is shown. Fuel A is supplied in any way suitable for the purpose under pressure by a pump 1 to the capillary test section 2. The test section 2 is e.g. clamped between two electrical bus bars (not shown for reasons of clarity) which supply current and allow the fuel in the test section to be heat stressed; the system is controlled to a constant fuel outlet temperature by any suitable fuel temperature control 3. B represents the outlet fuel stream. The narrow bore of the capillary (less than 300 μm, advantageously 250 μm) of the test section and its length of less than 200 mm, (advantageously 152 mm) ensures that at suitable fuel flow rates of e.g. 35 ml/min that the fuel is in the turbulent flow regime. The thermal degradation of test fuel within the test section is advantageously monitored in two ways: (1) A computer controlled scanning I.R. thermometer 4 measures outer wall temperatures along the test section 2 and subsequent increases in these temperature, promoted by the insulation effect of fuel degradation products; (2) Fuel degradation products are mostly carbon by mass, thus a carbon analyser (not shown) can assess the weight of carbon deposited. A fuel of low stability will produce a high weight of carbon deposition (e.g. 200 μg) and a high rise in outer wall temperature (e.g. 300 °C rise 10 mm from fuel outlet). A fuel of good stability will produce a much lower weight of carbon deposition (e.g. 10-20 μg) and a lower rise in outer wall temperature (e.g. 20 °C rise 10 mm from fuel outlet) . The ability to monitor accurately the amount of deposit within an extremely narrow bore has been achieved by scanning I.R. thermometry of the outer wall surface temperature. As fuel lacquer is deposited it insulates the hot metal from the colder fuel, thus the outer wall temperature increases during the test. Contact systems of temperature measurement on the test section (e.g. thermocouples) are not viable, either because of the large heat drain caused to the test section or alternatively damage to the small diameter-test section by attaching thermocouples. The use of an I.R. thermometer 4 enables the surface temperature of the test section 2 to be measured without contact. The test section 2 is e.g painted matt black to ensure a constant and high emissivity (close to 0.9). As already indicated in the foregoing, the fuel enters the test section 2 at a constant fuel inlet temperature and the heat applied to the test section 2 is controlled to a constant fuel outlet temperature via fuel temperature control 3 and any suitable temperature control program via connection line 5. As fuel deposits on the inside of the capillary it insulates the hotter metal from the colder fuel, i.e. it increases the thermal resistance at that point. The change in surface temperature from the start of test (ΔT) is a measure of the thermal resistance, which in turn is a measure of the fuel lacquer thickness. Thus, to determine the lacquer thickness within the capillary test section along its length, ΔT measurements are determined at several positions along the capillary test section. This is achieved by a computer (6) controlled system which scans by means of any suitable scanner control program via connection line 7 the I.R. thermometer 4 along the capillary 2 throughout the test, the temperature data so derived is logged on the same computer 6 throughout the test. The arrow C represents the scanner movement. Data D can be captured from the computer 6 and can be processed further in any way suitable for the purpose. E.g in this manner a "map" of the lacquer pattern along the length can be obtained, and this is a reliable way of distinguishing fuels of different stability. Such a "map" provides a three dimensional temperature profile of the capillary, resolved in position and time. Reference is made to fig. 2. In fig. 2 the vertical axis (Z) represents ΔT (in deg. C) , and the horizontal axes (X and Y) represent distance from the fuel outlet (in mm) and the time (in min.) respectively. The parts of different shading represent differences in profile temperatures. Each block of a particular shade represents the delta T (temperature) profile along the capillary for a given time, e.g. the first block at the front represents the delta T profile at zero time, the next shaded block immediately behind it represents the profile 10 minutes into the test. And so on, until the shaded block at the very back of the picture is the profile at the end of test (120 minutes in this example) . The method of the invention will now be illustrated by reference to the following example: The system uses advantageously a 254 micron (1/100 inch) diameter capillary with a pump delivering fuel at a rate of advantageously 35 ml/min. In practice, a 4.2 litre fuel sample is required in a 2 hour test. Along the capillary test section (advantageously 6 inches, 152 mm length) there is a marked fuel temperature gradient, fuel enters advantageously at ambient temperature (15-25 °C, in particular at 20 °C) and exits advantageously at 280-300 °C, in particular at 290 °C. There is thus a Reynold's number gradient along the test section due to the change in kinematic viscosity. At the fuel inlet the Reynold's number is circa 2500 and at the exit circa 14000, the average down the tube would be a Reynold's number of 8500 corresponding to a 170 °C fuel temperature. All monitoring of fuel deposition is advantageously within 21 mm of the fuel outlet, so only fuel deposition in the turbulent flow regime is followed. There is little scope for changing the capillary dimensions and yet at the same time maintain the use of a small sample and have measurable changes in heat transfer/thermal resistance: - For a given Reynold's number, halving the bore would increase the pressure drop across the test section from 350 PSI (about 25 bar) to 2800 PSI (about 196 bar) , it would be difficult to get a pumping system for these pressures. For a given Reynold's number, double the bore diameter would double the fuel sample to 8.4 litres, this is too large if the apparatus is to look at small samples from the field. The larger diameter would also mean that changes in heat transfer to measure deposit levels would be harder to discern. (Indeed they may not even be possible in the current system) . The scanning system software allows changes in the scanning routine but most of the fuel deposition is in the region 1-21 mm from the fuel outlet, so the scan advantageously reads a determined number of positions (e.g. nine) between 1 and 21 mm from the fuel outlet. Each scan of a single position includes a certain time wait (e.g. 10 seconds) (for stabilisation of the I.R. thermometer) then the computer averages a determined number of readings e.g ten over a determined number of seconds (e.g. ten) from this one position, before moving onto the next position. In this case all nine positions form the scan sequence which take circa 2.5 minutes to perform. A sequence of scans starts e.g. every five minutes, so in the 120 minute test there are 24 scan sequences. Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims: C L A I M S 1. A method for testing the thermal stability of a fuel comprising the steps of a) supplying under pressure a fuel sample to be tested to a small-bore capillary tube of a test section and allowing the fuel in the test section to be heat stressed; b) ensuring that the fuel sample in the test section is under turbulent flow regime; c) monitoring the thermal degradation of the test fuel within the section by measuring by means of contactless thermometry the outer wall temperatures along the test section and subsequent increases in these temperatures, promoted by the insulation effect of fuel degradation products; and d) deriving from the temperature data, thus obtained, information on the thermal stability of the fuel. 2. The method as claimed in claim 1, wherein the contactless thermometry is controlled scanning I.R. (infra-red) thermometry. 3. The method as claimed in claims 1 or 2, wherein fuel is deposited on the inside of the capillary tube at a determined distance from the fuel outlet, advantageously at 1-21 mm. 4. The method as claimed in claims 2 or 3, wherein the I.R. thermometry scan reads a determined number of positions, advantageously 9, along the capillary test section. 5. The method as claimed in claim 4, wherein each I.R. thermometry scan of a position includes a determined time wait, advantageously 10 sec. 6. The method as claimed in claim 5, wherein a determined number of readings (advantageously 10) are carried out over a determined number of seconds (advantageously 10) from one position before moving to the next position. 7. The method as claimed in any one of claims 2-6, wherein a sequence of scans is carried out. 8. The method as claimed in any one of claims 1-7, wherein the fuel sample consumption is less than 5 litre per test. 9. The method as claimed in any one of claims 1-8, comprising the step of assessing the weight of carbon deposit. 5 10. An apparatus for testing the thermal stability of a fuel comprising means for supplying under pressure a fuel sample to be tested to a small-bore capillary tube of a test section and means for allowing the fuel in the test section to be heat stressed; means for ensuring that the fuel sample in the test section is under turbulent flow regime; means for measuring by means of contactless thermometry the outer wall temperatures along the test section and subsequent increases in these temperatures, promoted by the insulation effect of fuel degradation products; and means for deriving from the temperature data, thus obtained, information on I- 3 the thermal 'stability of the fuel. 11. The apparatus as claimed in claim 10 wherein means for controlled scanning I.R. (infra-red) thermometry are present. 12. The apparatus as claimed in claim 10 or 11, wherein the capillary tube has a diameter of less than 300 μm. 20 13. The apparatus as claimed in any one of claims 10-12, wherein the capillary test section has a length of less than 200 mm. 14. The apparatus as claimed in any one of claims 10-13, wherein the fuel enters at 15-25 °C, in particular 20 °C and exits at 280-300 °C, in particular 290 °C. 25 15, The apparatus as claimed in any one of claims 10-14, wherein a carbon analyzer is present to assess the weight of carbon deposited.

Inventors: CLARK RICHARD HUGH, HARDWICK STEWART LEONARD, HOULBROOK GEOFFREY, KENDALL DAVID ROY, CLARK, RICHARD, HUGH, HARDWICK, STEWART, LEONARD, HOULBROOK, GEOFFREY, KENDALL, DAVID, ROY
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1994
Main classification: G01N33
IPC codes: G01N25, G01N33